A Rankine cycle system is described in Japanese Utility Model Application Laid-open No.59-174308, which includes an evaporator for heating a liquid-phase working medium by an exhaust gas from an internal combustion engine to generate a gas-phase working medium, an expander driven by the gas-phase working medium generated in the evaporator, a condenser for cooling the gas-phase working medium passed through the expander to restore it into the liquid-phase working medium, and a supply pump for supplying the liquid-phase working medium from the condenser in a pressurizing manner to the evaporator.
In the above-described conventional Rankine cycle system, the water as the liquid-phase working medium is passed not only through the inside of the evaporator mounted in an exhaust pipe from the internal combustion engine, but also through the inside of a cooling passage defined in a cylinder head and a cylinder block to heat them, whereby a waste heat from the internal combustion engine is utilized further effectively, and the cylinder head and the cylinder block are cooled by the liquid-phase working medium, thereby aiming at the disuse of a conventional radiator.
In general, however, the ratio of the flow rate of the water as the liquid-phase working medium in the Rankine cycle system to the flow rate of the cooling water for the internal combustion engine is on the order of 1:100, and thus, the flow rate of the cooling water for the internal combustion engine is far large, as compared with the flow rate of the water in the Rankine cycle system. The pressure of the water supplied to the evaporator in the Rankine cycle system is about 100 times the pressure of the cooling water supplied to the water jacket of the internal combustion engine, resulting in a large difference existing between both of the pressures.
Therefore, it is virtually difficult, because of a large difference in flow rate and pressure between the water-circulating paths, to connect a water-circulating path in the Rankine cycle system and a water-circulating path for the internal combustion engine in line to each other to aim at the disuse of a radiator, and there is a possibility that the internal combustion engine might be overheated and that the Rankine cycle system could not exhibit a sufficient performance.